My bike log has these metrics:Saddle height - centre of bb to top of saddle along the seat tubeSaddle fore-and-aft position (if the rails are graduated)Reach - centre of bars to middle of saddle railsSeat post offsetBar height - centre of front axle to top of bars (height of spacer stack may be useful too)Stem length. If I start to feel uncomfortable I quickly re-measure in case something has changed. Saddle rail or seat post slippage is not uncommon. Then I review any recent changes and consider if I should make a further change or revert to the baseline measurement. Any changes I make from my normal position are done in 5mm increments and logged. I also log maintenance items, like the date and odometer reading when I replace chains and cassettes. If you are into serious training then you need a training log too. To keep a note of things that your computer stats can't tell you, like the weather conditions, traffic, or how you were feeling e.g. your state of health and general well-being.

First up = I am in and have been in agony for the last 2 hours due to severe body soreness, back, numb hands. My bike is now unrideable until I rectify the fitment setup.

Second - I tries those interval speed regimes, and found it was a very different riding technique to what I normally ride, I was able to hold speed until the pain in my body set in real bad, and it was just a case of limping home in survival mode...

Don't bonk out in training rides imo.

Honestly if you want to do intervals, make it the focus of an hours riding, (ie warmup + 2x20 or warmup + 5x7) and then call it quits for the session. Hurting for me is running out of aerobic capacity towards the end of the effort, or legs unable to keep pushing so the average falls away. They aren't particularly sore, there is no joint pain, and nothing is numb, and my body isn't drained of resources by the effort to the extent that I go backwards instead of forwards.

After a 10 minute breather I can maintain usual cruising speed going home (because I am bike commuter, I need to ride back home again after finding somewhere trafficless to train), but I'd certainly warm down and stop after an interval.